Recently, thermoplastic resins having high heat resistance and high chemical resistance have been required as a material for the components of electric, electronic, automobile and chemical apparatus. In this regard, a polyarylene sulfide resins represented by polyphenylene sulfide represents one class resins which meets this requirement. However, polyarylene sulfide resins are disadvantageous in that the part formed by the fusion bonding of the heads of at least two melt flows of the resin joined in a mold cavity during molding (i.e., the weld) exhibits remarkably poor mechanical properties. Therefore, a polyarylene sulfide resin molded article containing a weld tends to break along the weld when subjected to thermal or mechanical stresses. Particularly, a polyarylene sulfide molded article having complicated shape produced by ultrasonic welding is more problematic in this regard. A polyarylene sulfide resin is a material suitable for precision molding, so that a molded article of the resin will necessarily contain a weld in most cases. Accordingly, many attempts have been made to improve the mechanical properties of the weld thereof, among which, the following techniques may be mentioned:
(1) improvement in mold design; for example, a mold having a cold-slug well is used and the corresponding part is cut off after the resulting molded article is cooled. Alternatively, the temperature drop of the material in the fusion bonding section is inhibited by increasing the number of gates or changing the positions of gates,
(2) necessary holes are formed by fabrication to inhibit the formation of a weld during molding,
(3) a composition having an improved fluidity is used (see Japanese Patent Laid-Open Nos. 11357/1984, 11358/1984 and 70157/1982) and
(4) an inorganic filler which effectively enhances the strength of the weld is used (see Japanese Patent Laid-Open No. 70157/1982).
However, these methods have the following problems. That is, according to the method (1), there are many welds which are unavoidable and thus the use of a cold-slug well lowers the production yield.
According to the method (2), the number of processing steps is increased which thereby increases the cost of the molded articles. Although the methods (3) and (4) appreciably improve the mechanical properties of the weld, the strength of the resulting weld is insufficient, so that the molded articles to which the methods can be applied are limited.
Thus, a polyarylene sulfide resin has been used only in limited fields in spite of its high heat resistance and its excellent precision moldability, because it gives a molded article containing welds that exhibit poor mechanical properties.